This invention relates to the preparation of aromatic compounds in a Grignard-type reaction. In a specific aspect, the invention relates to a catalyst for the preparation of bisbenzocyclobutenes.
Bisbenzocyclobutenes are monomers for the preparation of useful homopolymers and copolymers. Certain bisbenzocyclobutenes can be prepared by the reaction of benzocyclobutenyl Grignard reagent with a dihaloarene in the presence of a nickel or palladium catalyst. This type of coupling reaction is generally described, without reference to bisbenzocyclobutenes, in Kumada, Pure Appl. Chem. 52, 669-679 (1980).
It is known to use complexes of diphosphines, such as 1,3-bis(diphenylphosphino)propane, with nickel or palladium halides, particularly NiCl.sub.2, as catalysts for coupling Grignard reagents or other organometallics (such as organozinc reagents) to aromatic or vinyl halides. Although such diphosphine-nickel halide catalysts work extremely well in some coupling reactions, in others they are prone to "dieout", in which most or all of the catalyst becomes converted into an inactive species early during the reaction and the coupling stops from lack of active catalyst. In other types of reactions (such as Ullmann etherification) in which the "dieout" phenomenon occurs, it can be alleviated by slow addition of the catalyst over a long period of time. This insures that there is always at least a small amount of active catalyst present.
However, with the diphosphine-nickel halide complexes, continuous or incremental addition of the catalyst is difficult because the catalysts are insoluble in solvents, such as ethers, which are compatible with organometallic reagents. The catalysts therefore must be added to the reaction mixture as solids. It is difficult to measure repetitively small portions of solids under an inert atmosphere, especially an atmosphere saturated with solvent vapors. Addition pistons can sometimes be used, as can powder addition funnels using, for example, screw feeders. However, the latter devices tend to clog in atmospheres saturated with solvent vapors as the solid becomes wetted by solvent. Reasonably accurate incremental addition of solids, therefore, is difficult. It would be much more desirable to have a catalyst which is soluble in organometallic-compatible solvents such as ethers or hydrocarbons.
It is therefore an object of the invention to provide a catalyst and process for the reaction of an organometallic reagent and a reactive halide. In one aspect, it is an object of the invention to provide a soluble catalyst for the preparation of a bisbenzocyclobutene.